Dimmer as backup to Thermostat?

I have ordered my Herpstat but I do not have the cash to order a 2nd thermostat as a backup. This is my idea so please tell me what you think...

1. I plug my UTH into my Dimmer.
2. Set the dimmer level slightly higher than it needs to be to
achieve say 95 deg.
3. Plug this into the thermostat that is set to 92 deg.

With the Thermostat at full output, it should be able to achieve the target temp (92) while at the same it will be capped at 95 deg by dimmer. This would also solve the problem of the in tank temp probe getting water spilled on it or otherwise being cooled and the temp running wild.

Am I making sense to anyone but myself? What am I missing?

It would not work that way. Each device is going to limit the current (or voltage, depending upon how it works) that whatever is plugged into it receives.

If used together, the dimmer will be limiting the amount of electricity that the thermostat gets, and that is not what you want.

Additionally, a thermostat isn't an incredibly complex circuit, but most circuits are designed to operate at a certain level, and putting the dimmer in front is gonna change that.

Considering that dimmers affect the lifetime of a lightbulb, I've gotta think that it's going to have some affect on something like a thermostat.

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Originally Posted by ChrisBowsman

It would not work that way. Each device is going to limit the current (or voltage, depending upon how it works) that whatever is plugged into it receives.

If used together, the dimmer will be limiting the amount of electricity that the thermostat gets, and that is not what you want.

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I guess I just don't get it...its late and I am tired. Isn't the Dimmer after the Thermostat? So the dimmer would of course limit the current that the UTH receives, but how would it limit the supply of current comming into the dimmer via the thermostat?

I'm sorry... it's too early here :)

I thought you had the dimmer before the thermostat (in respect to the wall :)). Let me kick it around for a minute. I have to go to the dentist in a little while, so I'll ponder it while I'm there :)

My initial thought is still that two power control devices plugged in together could be problematic, but I could be wrong.

BTW, cool name for your normal ;) I watch A LOT of Pokemon thanks to my 2 & 5 yr olds.

Ok, I know it's only been a few minutes, but the more I think about it, it *should* work.

Do you have a multimeter? If I were trying something like this, I'd really want to test it before trying it with a snake. BTW, a multimeter would be a great investment. I realized my rheostat was faulty last night with a 30 second test with the meter rather than having to adjust it, wait for the temps to stabalize, test the temp again, adjust it, etc....

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Originally Posted by ChrisBowsman

BTW, cool name for your normal ;) I watch A LOT of Pokemon thanks to my 2 & 5 yr olds.

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Thanks, My daughter and I are huge pokemon fans and we named our snakes after them...Ekans is another snake pokemon (spelled "Snake" backwords :gj:)

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Originally Posted by ChrisBowsman

Ok, I know it's only been a few minutes, but the more I think about it, it *should* work.

Do you have a multimeter? If I were trying something like this, I'd really want to test it before trying it with a snake. BTW, a multimeter would be a great investment. I realized my rheostat was faulty last night with a 30 second test with the meter rather than having to adjust it, wait for the temps to stabalize, test the temp again, adjust it, etc....

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Great idea. How did exactly did you preform this test? I have a multimeter.

No it won't work that way you think it will. Think of the dimmer in terms of percentages...if you turn the dimmer down half way then it is only allowing half the power to get through.

(as an example we will use degrees)...say your thermostat is putting out 100 degrees this gets to the dimmer you have set at anything less then full (since the dimmer is power management not temp management) will decrease this 100 degrees down some percentage. If you increase the temp to say 110 the dimmer will still decrease the temp getting to the tub so in essence you aren't creating a safety you are having to increase the thermostat higher than necessary to compensate for what the dimmer is removing.

The only way to do what you want is with 2 thermostats (but the backup one can be a cheaper one...but it must measure the temps independent of the main thermostat otherwise how does it know the temp is too high)

Hope this helps

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Originally Posted by lord jackel

No it won't work that way you think it will. Think of the dimmer in terms of percentages...if you turn the dimmer down half way then it is only allowing half the power to get through.

(as an example we will use degrees)...say your thermostat is putting out 100 degrees this gets to the dimmer you have set at anything less then full (since the dimmer is power management not temp management) will decrease this 100 degrees down some percentage. If you increase the temp to say 110 the dimmer will still decrease the temp getting to the tub so in essence you aren't creating a safety you are having to increase the thermostat higher than necessary to compensate for what the dimmer is removing.

The only way to do what you want is with 2 thermostats (but the backup one can be a cheaper one...but it must measure the temps independent of the main thermostat otherwise how does it know the temp is too high)

Hope this helps

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Thanks for the reply, but I am still having trouble wrapping my head around this. Both of these devices (Dimmer and Tstat) are current limiting devices. Yes? No? I agree that the dimmer will decrease the power of whatever is input. And yes, the t-stat will increase power to compensate. Does it do this by reducing resistance? If so then will the t-stat at some point be 100% open as if it were not even there? This is what I an envisioning. The t-stat running at something like 98% open to achieve a temp of 92 deg. If it were to fail completely, then the dimmer would be getting 100% of the power (instead of 98%) as if the t-stat were not even there. In that case, the dimmer would still restrict the power to a safe level. I may just need a really dumbed down explanation.

You can get a cheap on off type T-stat for less than $40.00. Hook it in after the herpstat and have it set to a couple degrees higher than the herpstat. I don't know really what the issue is with just using the Herpstat as I don't know of any backups on the pro racks with 10 - 30 adult tubs using only one Helix! Most of these are designed to detect a fault and completely shut off their outputs. The probes on these are actually micro chips that can even detect if the probe develops a fault!

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Originally Posted by td30

With the Thermostat at full output, it should be able to achieve the target temp (92) while at the same it will be capped at 95 deg by dimmer.

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Don't do it. If you want a backup, keep the rheostat on hand in case the thermostat fails, don't try putting it inline.

As for why, a thermostat is just an electrically controlled rheostat, and as such, it's internal circuitry is only certified to operate at a specific voltage and amperage. By under-volting that circuitry, you can potentially damage it, and leave it incapable of controlling the rheostat, leading to possible fire hazards and increased risk to the animals being heated. Beyond that, you would be trying to keep a steady temperature based on a balancing act, and a slight change on either side (ie, bump the dimmer by accident) could lead to dangerously high temperatures.

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Originally Posted by ChrisBowsman

Ok, I know it's only been a few minutes, but the more I think about it, it *should* work.

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It might technically "work", but it will completely invalidate the ability of the thermostat to thermo-regulate correctly, will most likely cause damage to the thermostat itself, and will put the animals at risk.

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Originally Posted by td30

Both of these devices (Dimmer and Tstat) are current limiting devices.

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Yes, but one has internal circuitry designed to operate within a specific power range to limit the current.

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Originally Posted by td30

And yes, the t-stat will increase power to compensate. Does it do this by reducing resistance?

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Depends on the model of t-stat, but most simply switch the power on and off until the temperature is in range. In either case you can damage the circuitry that does the thermo-regulation, whether switched or metered, by under powering it.

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Originally Posted by td30

The t-stat running at something like 98% open to achieve a temp of 92 deg. If it were to fail completely, then the dimmer would be getting 100% of the power

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Most will shut down in case of a failure, an important feature to note when purchasing a thermostat. Also, most thermostats don't meter the power amount, but rather are "set" for target temperature ranges, which are measured for by the probe.

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Originally Posted by ctrlfreq

Most will shut down in case of a failure, an important feature to note when purchasing a thermostat. Also, most thermostats don't meter the power amount, but rather are "set" for target temperature ranges, which are measured for by the probe.

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This is why everyone should own a Herpstat.

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Originally Posted by ctrlfreq

As for why, a thermostat is just an electrically controlled rheostat, and as such, it's internal circuitry is only certified to operate at a specific voltage and amperage. By under-volting that circuitry, you can potentially damage it, and leave it incapable of controlling the rheostat, leading to possible fire hazards and increased risk to the animals being heated. Beyond that, you would be trying to keep a steady temperature based on a balancing act, and a slight change on either side (ie, bump the dimmer by accident) could lead to dangerously high temperatures.

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If the rheostat was going BEFORE the thermostat, I'd agree with you. However, it is AFTER, so it won't affect the amount of anything the thermostat sees.

The t-stat will be set to provide enough juice to heat the UTH to 92 degrees. If the r-stat is set to allow the passage of enough to heat it to 95, there is no problem.

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Originally Posted by Skoalbasher

This is why everyone should own a Herpstat.

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The Helix DBS-1000 has the same features. I have two Helix DBS-1000 T-stats that work great, I also have 2 Herpstat Pros and they work great also. I also use 3 Rancos on my incubators and have used them on tubs with no issues. All three of these are quality products in my opinion. I plan on trying Big Apples on off T-stat and proportional eventually, but I already see a problem in they do not have a digital temp read out showing what the probe is reading.

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Originally Posted by ChrisBowsman

If the rheostat was going BEFORE the thermostat, I'd agree with you. However, it is AFTER, so it won't affect the amount of anything the thermostat sees.

The t-stat will be set to provide enough juice to heat the UTH to 92 degrees. If the r-stat is set to allow the passage of enough to heat it to 95, there is no problem.

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The rheostat is a resister which limits current by using power! This means it will require the proportional T-stat to push more power to achieve the proper temps. This could result in overloading the proportional T-stat as most are only rated for between 400 Watts to 500 Watts. That equals a little less/over 4 amps at 120 Volts AC. In order to create a limiting point you will have to create a situation that limits based on its max Wattage. Running at max wattage is taxing on SCR's and transistors as they run hotter. This reduces the life of the proportional T-stat and also will create problems with the overload circuitry shutting down and no heat being created until it is reset. The only way I would do it is use a On/Off T-stat before the proportional controller if a back up was needed!

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Originally Posted by ChrisBowsman

The t-stat will be set to provide enough juice to heat the UTH to 92 degrees. If the r-stat is set to allow the passage of enough to heat it to 95, there is no problem.

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First, if the thermostat is set to heat to 92 degrees, you will never see 95 degrees regardless of whether there is a rheostat inline or not. The thermostat is going to turn on and off based on the probe temperature. The risk is now the fact that by introducing another ~12 feet of wire (the length of the run to and from the inline rheostat), and it's associated resistance, the power draw against the thermostat is going to be greater than it needs to be, and which can shorten it's lifetime considerably.

In either case, the answer is simply to make sure the thermostat will shut down in the case of a problem, and to have a rheostat on-hand to replace it with temporarily if needed. Anything else will shorten the lifespan of the equipment, and potentially put the animals in unnecessary risk.

As a side note and stating a situation where backups are a good idea I would like to bring up how I wired my incubator. I see a lot of people using 1 T-stat or such on simple home made incubators. While if you are incubating normal ball pythons or such this is a readily acceptable way of doing it. Or if you are just playing around to see what you can do it works pretty good and is an inexpensive way of getting the job done. Now when you get into high end morphs that have the potential to sell for over a grand apiece is the risk to even one egg worth the 3 to 4 hundred dollars saved? I personally bought a professional incubator that was recommended by one of the more notable breeders that we purchased ball pythons from, Michael Cole of Ballroom Pythons South. He uses the dual wafer type T-stats as a primary and back up, but also backs all this up with a Ranco as the ultimate back up that shuts everything down now if 91 degrees is reached inside. I took this one step further and replaced both wafer T-stats with rancos and then added in the third as a full blown emergency shut down. The primary is set to 88 degrees the secondary is at 89 degrees and the master over temp one is set to 91 degrees. So if the 88 degree one fails the 89 degree one will take over as normally closed relay contacts are in use on the ranco if the 89 degree one fails the 91 degree one will start to cycle the whole unit including the fan which the 88 and 89 degree T-stats have no control over. I really like this triple back up idea on an incubator as it may run 3 to 5 days at a time without getting more attention than a glance, where as our snakes have their water changed daily meaning they are inspected daily and heat gunned daily! When you consider the cost of the whole set up at $650.00 and it can hold 9 clutches of eggs compared to the cost of loosing one Bumble Bee morph before it even hatches $650.00 becomes pocket change really quick. This is probably one of the only cases I would really consider using a back up for. At most on my snakes I may install a Hi/Lo temp alarm on them to warn me if the temps get out of range and let me know action is needed!

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Originally Posted by td30

Thanks for the reply, but I am still having trouble wrapping my head around this. Both of these devices (Dimmer and Tstat) are current limiting devices. Yes? No? I agree that the dimmer will decrease the power of whatever is input. And yes, the t-stat will increase power to compensate. Does it do this by reducing resistance? If so then will the t-stat at some point be 100% open as if it were not even there? This is what I an envisioning. The t-stat running at something like 98% open to achieve a temp of 92 deg. If it were to fail completely, then the dimmer would be getting 100% of the power (instead of 98%) as if the t-stat were not even there. In that case, the dimmer would still restrict the power to a safe level. I may just need a really dumbed down explanation.

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Ok...let me try again to explain why it won't work...A dimmer does not measure temp so it has no idea when the thermostat is acting up and sending too much power. Back to my example: if the thermostat is outputting 100 degrees. First how do you set the dimmer at 105?? (you can't..it is impossible cause there is no way to tell it is set at 105 degrees - as the other thermostat is only outputting 100 and the dimmer cannot tell temps). There is nothing you can do with a dimmer when used with a thermostat (either before or after the thermostat...they work differently)

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Originally Posted by ctrlfreq

First, if the thermostat is set to heat to 92 degrees, you will never see 95 degrees regardless of whether there is a rheostat inline or not.

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I think the idea for the rheostat being there was in case the thermostat shorted.

I was just thinking about it from a really basic standpoint, and will digress to those with more knowledge of the inner workings of thermostats :)

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Originally Posted by ChrisBowsman

I think the idea for the rheostat being there was in case the thermostat shorted.

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I can understand that, but the problem is, if the rheostat is inline with a thermostat, and the thermostat fails, with any decent thermostat, there will be no current on the line after the failure. If you're looking to have a true backup, it will need to be parallel to the t-stat circuit, and not in series.

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Originally Posted by ctrlfreq

If you're looking to have a true backup, it will need to be parallel to the t-stat circuit, and not in series.

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I didn't know that either. If I had known that they shut down, I'd never have thought it would work in the first place (obviously).

Just as an example.

I have one of the older model Herpstat II's, and it came with the original white, flimsy probes instead of the heavy-duty grey probes. One night, one of the probes failed(at 2am no less), either by being bent or it just failed.. Either way, that thermostat shut that section off and made this high pitched wailing alarm. It was quite obvious that it didn't need help shutting itself off or regulating itself. After about a month, the other probe failed too, but the unit shut that section of the thermostat off, and beeped at me. At least now those t-stats come with the heavy-duty probes and it's unlikely that the probes will fail again. If they do, I trust the unit to take care of itself.

If I did want to back it up, all I would have to do is get a Johnson Controls or Ranco, plug it in to the wall, set it about 5 degrees higher than what you want your hotspot at. Then plug your Helix or Herpstat into the JC or Ranco, set it to what you want your hotspot to be, and that's it.

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Originally Posted by ctrlfreq

I can understand that, but the problem is, if the rheostat is inline with a thermostat, and the thermostat fails, with any decent thermostat, there will be no current on the line after the failure. If you're looking to have a true backup, it will need to be parallel to the t-stat circuit, and not in series.

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Thanks everyone for the replys. This makes the mose sense to me. Glad I ordered the Herpstat:gj:

The people who insist that a rheostat won't work after a thermostat are mistaken.
If that were the case, a thermostat could only be used with one particular type of heat tape. By adding the rheostat, as far as the thermostat is concerned, you're just changing the characteristics of the heat tape.
Say the heat tape would normally get to 120F with the thermostat at 100%. You could set up initially without the thermostat and adjust the rheostat so the tape is 95F when the room is at the coldest. Then you add the thermostat before the rheostat and set it to the desired temp. If the thermostat fails 100% on, your heat tape won't be able to go over 95F. If it fails 100% off, your snakes will drop to room temp, but that's a lot better than cooking them.

The better solution is to use 4" flexwatt, which is designed to run cooler, to avoid the whole problem. I wouldn't use anything but a herpstat to regulate the temps.

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Originally Posted by JeffFlanagan

If that were the case, a thermostat could only be used with one particular type of heat tape. By adding the rheostat, as far as the thermostat is concerned, you're just changing the characteristics of the heat tape.

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That's kind of what I was thinking.

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Originally Posted by JeffFlanagan

The people who insist that a rheostat won't work after a thermostat are mistaken.
If that were the case, a thermostat could only be used with one particular type of heat tape. By adding the rheostat, as far as the thermostat is concerned, you're just changing the characteristics of the heat tape.
Say the heat tape would normally get to 120F with the thermostat at 100%. You could set up initially without the thermostat and adjust the rheostat so the tape is 95F when the room is at the coldest. Then you add the thermostat before the rheostat and set it to the desired temp. If the thermostat fails 100% on, your heat tape won't be able to go over 95F. If it fails 100% off, your snakes will drop to room temp, but that's a lot better than cooking them.

The better solution is to use 4" flexwatt, which is designed to run cooler, to avoid the whole problem. I wouldn't use anything but a herpstat to regulate the temps.

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Originally Posted by Gloryhound

The rheostat is a resister which limits current by using power! This means it will require the proportional T-stat to push more power to achieve the proper temps. This could result in overloading the proportional T-stat as most are only rated for between 400 Watts to 500 Watts. That equals a little less/over 4 amps at 120 Volts AC. In order to create a limiting point you will have to create a situation that limits based on its max Wattage. Running at max wattage is taxing on SCR's and transistors as they run hotter. This reduces the life of the proportional T-stat and also will create problems with the overload circuitry shutting down and no heat being created until it is reset. The only way I would do it is use a On/Off T-stat before the proportional controller if a back up was needed!

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You can't hook over a 400 Watt heating element to a herpstat Pro! That is the most it will push!
Watts = Amps X Voltage
Voltage = Amps X Resistance

If you increase the resistance in the circuit voltage will go up to maintain a given amount of Amps through the circuit. If Voltage goes up to keep Amps the same then Watts go up! More Watts a proportional controller pushes out the hotter the internal circuitry will get. The hotter the internal circuitry gets the more likely failure becomes.

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Originally Posted by Gloryhound

If you increase the resistance in the circuit voltage will go up to maintain a given amount of Amps through the circuit. If Voltage goes up to keep Amps the same then Watts go up! More Watts a proportional controller pushes out the hotter the internal circuitry will get. The hotter the internal circuitry gets the more likely failure becomes.

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If someone put a rheostat in parallel with the heat tape, a bad mistake, then your concerns would be well-founded.
In series, it's limiting the amount of current that could go through the thermostat. For example if you had 10W tape, with no rheostat and the probe came loose, you'd be pulling 10W through the thermostat at 110v. If you reduce the current 50% with a rheostat, the tape would only be able to pull 5W at 110v. As far as the thermostat knows you just replaced your 10w tape with 5w

The thermostat is going to run at a higher % at all times with the rheostat in place, but that current is going to be limited by the resistor in the circuit, and will use less power at max output than the tape alone. I'm not aware of any problem with a herpstat running around 100% for extended periods as long as you're not drawing too much current.

I don't think it's as good a plan as just using lower wattage heat tape, but it could work. A non-proportional thermostat before the proportional one may be a better idea, but that would increase your odds of a no-heat failure state.

I recommend 4" flexwatt + a herpstat, and no screwing around with rheostats or second thermostats at all.

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Originally Posted by JeffFlanagan

If someone put a rheostat in parallel with the heat tape, a bad mistake, then your concerns would be well-founded.
In series, it's limiting the amount of current that could go through the thermostat. For example if you had 10W tape, with no rheostat and the probe came loose, you'd be pulling 10W through the thermostat at 110v. If you reduce the current 50% with a rheostat, the tape would only be able to pull 5W at 110v. As far as the thermostat knows you just replaced your 10w tape with 5w

The thermostat is going to run at a higher % at all times with the rheostat in place, but that current is going to be limited by the resistor in the circuit, and will use less power at max output than the tape alone. I'm not aware of any problem with a herpstat running around 100% for extended periods as long as you're not drawing too much current.

I don't think it's as good a plan as just using lower wattage heat tape, but it could work. A non-proportional thermostat before the proportional one may be a better idea, but that would increase your odds of a no-heat failure state.

I recommend 4" flexwatt + a herpstat, and no screwing around with rheostats or second thermostats at all.

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Current is a constant in a series circuit and voltage is divided by the various loads.

Voltage is a constant in a Parallel circuit and the current is divided by the various loads.

This is why all household recepticles and lights are wired in parallel. If not each divice would not see 110V AC.

After this question really bothering me last night I had to sit down and do all the math and draw myself many pictures plugging in various resistances. After doing all this I have come to the conclusion that the rheostat in series after the proportional controller would work! I still do not believe it should be done though as it has the potential to become more probmatic than just using the proportional and will slow down the reaction time of the proportional controller possibly causing the temps to vary almost twice as much as it would with just the proportional controller. Here is the math that makes it work.

All numbers are made up just to use as an example.

UTH = Resistance of 100 Ohms

Rheostat = variable resistance of 0 to 200 Ohms, but for the primary example we will use a setting of 100 Ohms to start out.

Max Voltage that can be applied = 120 VAC

That means with just the UTH you would have a current of 1.2 Amps going through the UTH.
Amps = Voltage / Resistance
1.2 Amps = 120 VAC (Max AC voltage that can be applied) / 100 Ohms (UTH Resistance)
Thus max watts on UTH would be 144 Watts.
Watts = Volts X Amps
144 Watts = 120 Volts X 1.2 Amps


Add in the Rheostat set at 100 Ohms and the resistance of the circuit doubles to a total of 200 Ohms. Max current through the circuit gets cut in half to 0.6 Amps. Also the Max Voltage the UTH will see would be cut in half as each of the resistances would drop 1/2 the applied max current through it.
Amps = Volts / Total resistance of circuit
0.6 Amps = 120 Volts / 200 Ohms
Watts = Volts X Amps
72 Watts = 120 Volts X 0.6 Amps

Now in a series circuit the voltage drops across each of the resistances proportionally to the amount of the resistance. In this example the resistance are equal, so 60 Volts would be dropped across each load. Also in a series circuit the Current is a constant so each load will see the full 0.6 Amps of current flowing through it. This means each load will have to dissipate 36 Watts of energy.
Watts = Volts X Amps
36 Watts = 60 Volts (voltage used by each individual load) X 0.6 Amps

Now for the more advanced let us set the rheostat at 150 Ohms.
Amps = total applied Voltage (120 VAC) / total resistance of the circuit (100 Ohm for UTH + 150 Ohms for rheostat = 250 Ohms)
0.48 Amps = 120 VAC / 250 Ohms
Total Watts Used = Applied VAC X Amps
57.6 Watts = 120 VAC X 0.48 Amps

Now the voltage is not equally divided between the two loads since they have different resistances so we have to figure out what Voltage each load drops by using Proportions.

120 VAC / 250 Ohms = Y / 150 Ohms (Resistance of Rheostat)
0.48 Amps = Y / 150 Ohms
0.48 Amps X 150 Ohms = Y (We are back to basic Ohms Law)
72 VAC = Y
Now 72 VAC X 0.48 Amps = 34.56 Watts

Now lets do the UTH
120 VAC / 250 Ohms = Y / 100 Ohms (Resistance of UTH)
0.48 Amps = Y / 100 Ohms
0.48 Amps X 100 Ohms = Y
48 Volts = Y
Then 48 VAC X 0.48 Amps = 23.04 Watts

Lets double check.
Do the voltages dropped across each load equal the total applied voltage?
72 VAC (for Rheostat) + 48 VAC (For UTH) = 120 VAC :gj:
Does the total Watts used equal the Watts used by each load?
34.56 Watts + 23.04 Watts = 57.6 :gj:

OK! I confess, I am an electroncis geek! :tears:

I still disagree...as to how it relates to the use as a backup. Firstly how to you know precisely what the rheostat is set at (it is only a rotary dimmer and has NO WAY to monitor any temp or wattage change coming to it). It is still limiting the output coming through it as a percentage of the total power being applied to it.

So you apply 100 watts from the thermostat and dial in the dimmer so it achieves a 92 degree temp (you could even argue that you increase the dimmer slightly so as to give some unknown amount of overhead - but again you can't be sure what that is as there is no way to be sure).

So you set it up as you describe...but then the thermostat breaks and locks open applying the full 500 watts (what most thermostats max out at) to the system so the dimmer will only limit this 500 watts to what?? It will only limit it as a percentage so even it the dimmer was turned down to 50% you would still be applying 250 watts to the heat tape...which is plenty to cook your snakes.

Again a dimmer won't work as a back-up to a thermostat.

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Originally Posted by lord jackel

I still disagree...as to how it relates to the use as a backup. Firstly how to you know precisely what the rheostat is set at (it is only a rotary dimmer and has NO WAY to monitor any temp or wattage change coming to it). It is still limiting the output coming through it as a percentage of the total power being applied to it.

So you apply 100 watts from the thermostat and dial in the dimmer so it achieves a 92 degree temp (you could even argue that you increase the dimmer slightly so as to give some unknown amount of overhead - but again you can't be sure what that is as there is no way to be sure).

So you set it up as you describe...but then the thermostat breaks and locks open applying the full 500 watts (what most thermostats max out at) to the system so the dimmer will only limit this 500 watts to what?? It will only limit it as a percentage so even it the dimmer was turned down to 50% you would still be applying 250 watts to the heat tape...which is plenty to cook your snakes.

Again a dimmer won't work as a back-up to a thermostat.

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Don't get me wrong either I don't like the idea of doing this as a back up either and believe if you really felt you needed the security of a back up an on off type Thermostat is the best way to go!

But as for the math and science it should work. Also you have to remember the resistances are not changing and the proportional T-stat has a max voltage output. Even though the Proportiona T-stat is capable of supplying 400 Watts of power it can only supply 120 VAC or some other voltage lower depending on manufacturer. The resistance of the heat mat is based on a given voltage (Normally 120 VAC, 110 VAC, and 220 VAC if in europe.) So say it is rated for 20 Watts Amps = Watts/Voltage
Amps = 20 Watts / 120 Volts = 0.166666666 Amps.
Resistance = Volts / Amps or 120 Volts / 0.166666666 = 720 Ohms. The value of resistance is not going to change regardless of the actual applied voltage. So the Proportion T-stat will not be able to push its full rated watts out through this singel load. You have to have multiple heat strips in parallel to get close to the max watt rating of the controller.

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Originally Posted by Gloryhound

After doing all this I have come to the conclusion that the rheostat in series after the proportional controller would work!

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I'm not sure the question was really whether or not it "could" be done, but whether it "should" be done given the potential risk to equipment, heating elements, and animals being heated.

Sure, you can super-glue yourself to the seat of your car so you won't go flying around in the case of an accident, but why would you when almost all cars have such nice seat-belts already, and the super-glue will most likely ruin your clothes ;)

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Originally Posted by Gloryhound

Don't get me wrong either I don't like the idea of doing this as a back up either and believe if you really felt you needed the security of a back up an on off type Thermostat is the best way to go!

But as for the math and science it should work. Also you have to remember the resistances are not changing and the proportional T-stat has a max voltage output. Even though the Proportiona T-stat is capable of supplying 400 Watts of power it can only supply 120 VAC or some other voltage lower depending on manufacturer. The resistance of the heat mat is based on a given voltage (Normally 120 VAC, 110 VAC, and 220 VAC if in europe.) So say it is rated for 20 Watts Amps = Watts/Voltage
Amps = 20 Watts / 120 Volts = 0.166666666 Amps.
Resistance = Volts / Amps or 120 Volts / 0.166666666 = 720 Ohms. The value of resistance is not going to change regardless of the actual applied voltage. So the Proportion T-stat will not be able to push its full rated watts out through this singel load. You have to have multiple heat strips in parallel to get close to the max watt rating of the controller.

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But using this same logic doesn't this mean that you really don't need a back-up at all since even if the thermostat is stuck open it cannot push enough current to cause an issue?

A Rheostat by design lowers the amount of power getting through it...so if it blocks 10% then is blocks 10% of 1000watts or 10watts - the difference is still huge. This is the same reason why the actual temp in the tub fluctuates with the ambient temp of the room when using a rheostat cause it cannot compensate for increase or decreases in temp.

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Originally Posted by lord jackel

But using this same logic doesn't this mean that you really don't need a back-up at all since even if the thermostat is stuck open it cannot push enough current to cause an issue?

A Rheostat by design lowers the amount of power getting through it...so if it blocks 10% then is blocks 10% of 1000watts or 10watts - the difference is still huge. This is the same reason why the actual temp in the tub fluctuates with the ambient temp of the room when using a rheostat cause it cannot compensate for increase or decreases in temp.

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Absolutely correct and why I stated that while the circuit would work with the rheostat, I still do not recommend it! You would to clamp a UTH at a certain wattage you would have to calculate its resistance through the use of the Rated Voltage and Wattage of the UTH. Then calculate a resistance to put in series with that that would create a situation in which only a given voltage that would keep the wattage from going past a given point. At this point you would use a fixed type resister in series with the UTH to accomplish this. Still not the way I would do it, but it is a way to accomplish a means! But to figure out what wattage you want the heater to run at you would need to get into transfering Watts to BTU's and calculating volume of the enclosure and a whole bunch of other variables such as temp of room the enclosure is in, which in most houses will not stay constant throughout the year. Back to constantly making adjustments on your rheostat. Not fun!

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Originally Posted by Gloryhound

Absolutely correct and why I stated that while the circuit would work with the rheostat, I still do not recommend it! You would to clamp a UTH at a certain wattage you would have to calculate its resistance through the use of the Rated Voltage and Wattage of the UTH. Then calculate a resistance to put in series with that that would create a situation in which only a given voltage that would keep the wattage from going past a given point. At this point you would use a fixed type resister in series with the UTH to accomplish this. Still not the way I would do it, but it is a way to accomplish a means! But to figure out what wattage you want the heater to run at you would need to get into transfering Watts to BTU's and calculating volume of the enclosure and a whole bunch of other variables such as temp of room the enclosure is in, which in most houses will not stay constant throughout the year. Back to constantly making adjustments on your rheostat. Not fun!

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Oh good...I thought we were dancing around the same answer from different points of view (or to different ways of asking the question).

So in short for everyone following this -
Would the circuit work? - YES.
Should it be used as a backup - NO

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Originally Posted by lord jackel

Oh good...I thought we were dancing around the same answer from different points of view (or to different ways of asking the question).

So in short for everyone following this -
Would the circuit work? - YES.
Should it be used as a backup - NO

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I second this motion! All in favor say "aye"! :gj:

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Originally Posted by lord jackel

Oh good...I thought we were dancing around the same answer from different points of view (or to different ways of asking the question).

So in short for everyone following this -
Would the circuit work? - YES.
Should it be used as a backup - NO

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Originally Posted by Gloryhound

I second this motion! All in favor say "aye"! :gj:

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Thanks for helping to figure this out. I guess I never thought it would be the best way to do it, but it made sense in my head and I was hoping it would work. So needless to say, I won't risk damaging my new t-stat (or snake) to test it.